A storage battery for domestic or industrial use has a problem that a storage capacity of the storage battery at a full charge is decreased by repeatedly charging and discharging. Accordingly, a technique for calculating an actual capacity of a storage battery has been proposed.
PTL 1 calculates an actual capacity, based on a charge capacity obtained from a completely discharged state to a fully charged state. At this time, it is determined that the completely discharged state is reached when discharge voltage becomes a predetermined value (e.g., about 3V).